1. Field of the Invention
The present invention relates to a communication system that supports reservation of network resources, and arts related thereto.
2. Description of the Related Art
In communications in network environments such as the Internet, data is decomposed to a plurality of packets, which are transmitted via the network.
In general, the transmission of packets is processed in a “best effort” mode. This means that traffic of AV (Audio/Visual) communications and traffic of other transmissions, for example ftp/http transmissions, are treated equally, although what should be processed in real time is not the traffic of other transmissions but the traffic of AV communications.
Therefore, when the network is crowded with the traffic of other transmissions, sound/music may break into pieces or quality of moving-picture may be deteriorated.
In order to communicate multi-media data, such as AV data, without quality deterioration, it is necessary to reserve network resources using reservation on a network path to guarantee communication quality.
The IETF (Internet Engineering Task Force) has defined resource management protocol, such as RSVP (Resource Reservation Protocol), as a method of reserving network resources.
In RSVP, network resources required for multi-media communications are allocated on a communication path from one terminal to a communication partner thereof, before the multi-media communications begin.
When RSVP is used, since the network resources required for every stream can become reservable before the communications begin, communication quality can be guaranteed.
SBM (Subnet Bandwidth Manager) is protocol for admission control and bandwidth management on IEEE802.1LAN, which is based on RSVP. SBM realizes bandwidth reservation in link layers, using bandwidth management functions called DSBM (Designated Subnet Bandwidth Manager).
Hereinafter, operation using RSVP will now be explained. In RSVP, bandwidth reservation is performed by transmitting admission control messages among a network-relaying device that supports RSVP, a transmitting terminal and a receiving terminal. The transmitting terminals transmit a PATH message that describes traffic properties of transmitting data to the receiving terminal.
The PATH message reaches the receiving terminal via a path composed of the network-relaying device. The receiving terminal transmits to the transmitting terminal a RESV message that describes network resources required for receiving. The network devices on the path reserve their own network resources according to the contents of the RESV message, thereby, the bandwidth reservation of the communication between the transmitting terminal and the receiving terminal is realized. Transmitting a RESV message periodically can continue the reservation of the resources.
On the other hand, DiffServ (Differenciated Services) is defined in the IETF as a bandwidth reservation method based on reservation of network resources. DiffServ belongs to priority control type protocol. When DiffServ is used in a DS field of an IP header, the priority value of DSCP (DiffServCodePoint) is set corresponding to priority class classified according to significance of data.
The network-relaying device on the network can identify priority based on this DSCP value, and can transmit packets with a higher priority prior to packets with other classes of priority, while relating to the network resources reserved by RSVP.
IEEE802.1p is defined as a method for realizing priority control in a “layer 2” level. IEEE802.1 p may be used in a switch having priority control functions.
Next, an example of bandwidth reservation is explained. A terminal reserves network resources of network devices existing on a communication path using RSVP. The network-relaying device uses a value of DSCP of an IP header, or a value of IEEE802.1p of an IEEE802.1Q VLAN header, in order to identify the reservation.
For example, a router, which belongs to the network relaying device, is identified as follows: Packets for each of which DSCP value is “5” belong to a flow whose assigned bandwidth is 30 Mbps.
A switch, which also belongs to the network relaying device, is identified as follows: Packets for each of which IEEE802.1p value is “6” belong to a flow that should be processed in real time.
On the other hand, the transmitting terminal transmits packets after setting a DSCP value to an IP header of each of the packets and setting an IEEE802.1p priority to a MAC header of each of the packets. The network relaying device can perform priority control based on DSCP and/or the IEEE802.1p priority. Thus, a flow that priority should be set is separated from other flow not requiring priority be set, thereby guaranteeing bandwidth.
In the conventional techniques (RSVP and/or SBM), the reservation of resources is performed by transmitting packets for reservation when service begins and sending reservation messages periodically to maintain the reservation.
However, as described below, with the conventional techniques, when a terminal moves and the communication path is changed, communication quality cannot be guaranteed.
(When a Source Terminal Moves)
FIG. 13 is a block diagram of a conventional communication system. In this communication system, each of base stations 101, 102 and 103 plays a role of the relaying device. The base station 103 and the base station 101, and the base station 103 and the base station 102, respectively, are connected by a cable and/or wireless.
A terminal 140 connects to the base station 103, and terminals 110, 120, and 130 connect to the base station 102.
There is a communication path 200. In the communication path 200, the terminal 140 is a source terminal, and the communication path 200 continues, via the base stations 103 and 102, to the terminal 110 that is a destination terminal. The network resources of the communication path 200 are reserved.
There is a communication path 201. In the communication path 201, the terminal 120 is a source terminal, and the communication path 201 continues, via the base station 102, to the terminal 130 that is a destination terminal. The network resources of the communication path 201 are reserved.
As shown using an arrow N1 of FIG. 14, since the terminal 120 has moved, a connection between the terminal 120 and the base station 102 is canceled, and the terminal 120 connects with the base station 101. Consequently, a new communication path 202 is formed. In the communication path 202, the terminal 120 is a source terminal, and the communication path 202 continues, via the base stations 101, 102 and 103, to the terminal 130 that is a destination terminal.
However, when the communication path 202 has just been formed, the network resources of the communication path 202 have not been reserved yet.
If the terminal 120 continues to transmit packets after moving, the terminal 120 transmits packets without reservation until the reservation of communication path 202 is established, while the base stations 101, 103, and 102 perform priority control of packets belonging to a flow that is not reserved.
Consequently, since the bandwidth of the traffic of communication path 200 to which priority should be set is essentially suppressed, the communication quality of the traffic may deteriorate.
(When a Destination Terminal Moves)
FIG. 15 is a block diagram of a conventional communication system. In the state of FIG. 15, the terminal 140 connects to the base station 103, the terminal 110 connects to the base station 101, and the terminals 120 and 130 connect to the base station 102.
There is a communication path 203. In the communication path 203, the terminal 140 is a source terminal, and the communication path 203 continues, via the base stations 103 and 101, to the terminal 110 that is a destination terminal. The network resources of the communication path 203 are reserved.
There is a communication path 204. In the communication path 204, the terminal 140 is a source terminal, and the communication path 204 continues, via the base stations 103 and 102, to the terminal 120 that is a destination terminal. The network resources of the communication path 204 are reserved.
As shown using an arrow N2 of FIG. 16, since the terminal 120 has moved, a connection between the terminal 120 and the base station 102 is canceled and the terminal 120 connects with the base station 101. Consequently, a new communication path 205 is formed.
Like the above-mentioned case, if the terminal 140 continues to transmit packets after the terminal 120 moves, the bandwidth of the traffic of communication path 203 to which priority should be set is essentially suppressed, the communication quality of the traffic may deteriorate.
In recent years, since wireless LANs and mobile environment spread far and wide, it is obvious that communication paths frequently change according to moving of a terminal and solution thereof is required.